1. Field of the Invention
This invention relates to a pneumatic radial tire for a small-size passenger car having improved steering stability by improving a structure of a bead portion without causing the lowering of tire breaking pressure (safety factor of tire) resulting from the bead portion and capable of reducing the cost and improving production efficiency by controlling an extra use of bead wire.
2. Description of the Related Art
In this type of the pneumatic tire, the bead core located in the bead portion acts to fix an end portion of the carcass ply comprised of fiber cords or steel wire cords wound therearound and to define a size of an inner periphery of the tire itself and ensure fitting to a wheel rim.
Thus, the bead core is a very important member in a pneumatic tire, so that there have been proposed various structures of the bead portion.
For instance, a known bead core is obtained by spirally winding a band-shaped rubber-wire composite containing plural wires arranged side by side and embedded therein several times into a given shape as shown in FIGS. 1 and 2 (JP-U-61-110406) and also, a monostrand bead core for use in heavy duty pneumatic tires obtained by coating a wire with uncured rubber at a given thickness to from a bead wire and then spirally winding the bead wire in a given shape (JP-A-2-286407).
In the conventional pneumatic tire, however, a relation between a length overlapping a winding start portion with a winding terminal portion in the bead core and a tire breaking pressure (safety factor of tire) is not clear, so that the tire breaking pressure in the bead portion is insufficient. In particular, the stress concentration is increased in the step portion defined by the winding start portion or the winding terminal portion to bring about the lowering of the tire breaking pressure. Furthermore, the amount of bead wire used is excessive to increase the cost and degrade the production efficiency.
Moreover, the shape of the bead core made from the band-shaped rubber-wire composite or the monostrand bead core as mentioned above is largely obtained in tire building and vulcanization steps, so that it is difficult to hold tire uniformity in circumferential and radial directions of the tire and hence the steering stability of the resulting tire product is considerably degraded.